Microporous membranes have been prepared for several years. U.S. Pat. No. 4,340,479 generally describes the preparation of skinless microporous membranes by casting a polymer resin solution onto a substrate and quenching the resulting thin film of polymer.
While these membranes are suitable for a variety of purposes, they suffer from several disadvantages. In particular, such membranes are relatively fragile. In order to impart mechanical strength to such membranes, they are usually mated to nonwoven fibrous support material; however, casting the polymer resin solution onto such material is not without difficulties and problems. The substrate preferably has a large pore size to minimize the pressure drop across the supported membrane and coarse fibers to provide the greatest mechanical strength. Substrate pores which are too large, however, lead to gaps or holes in the membrane coating on the support, and increasing fiber coarseness results in increasing fiber stiffness and the possibility of membrane damage during the physical manipulations of the supported membrane needed to prepare, for example, a filter element such as a filter cartridge.
Moreover, in the coating process, there are invariably fibers extending from the main mass of the fibers which form the support material. Thus, the membrane is not formed upon an entirely smooth surface, and the thickness of the membrane layer must be increased to ensure that all such extending fibers and any defects in the membrane layer introduced by such extending fibers are completely covered by sufficient unbroken membrane to yield the desired filtration characteristics.
Attempts at avoiding some of these problems by separately forming the membrane and then laminating it to a suitable support material, typically by the application of heat thereto, have not been entirely successful. While lamination is capable of adequately addressing many of the structural and processing deficiencies of microporous membranes, other problems are introduced by this methodology. The most significant problem is the potential for subsequent delamination of the membrane. This problem is of particular concern when a membrane is cleaned for reuse by backflushing the filtration system. Another problem with lamination relates to the potential effect the lamination procedure has upon the pore structure of the membrane. When heat is used to effect lamination, the increase in temperature can damage the membrane by altering the structure or pore size of the membrane at its surface. Such pore size alterations can affect the resolution and/or useful life of the membrane. As a result, laminated microporous membranes are not entirely desirable, and membranes formed directly on suitable support materials are more typically used, albeit with certain compromises.
In order to compensate for as many of the aforementioned problems as possible, commercially available skinless microporous membranes generally utilize a relatively thick nonwoven fibrous support material with fine pores and fine fibers with a membrane layer of substantial thickness encompassing the entire support material, i.e., the support material is entirely embedded in the membrane. The resulting supported microporous membrane is generally satisfactory for its intended purposes but is rather thick and exhibits a high pressure drop. Moreover, in critical applications, such as in removing bacteria, viruses, and other harmful contaminants from pharmaceutical products, two such microporous membranes are typically used in series to ensure that the expected removal of the contaminants, referred to as titer reduction, is actually achieved. The use of two or more membranes in series, however, results in significantly higher pressure drops.
There has been some effort at preparing skinned microporous membranes which differ from the aforementioned skinless membranes in having a dense skin penetrated by pores of smaller diameter than the pores in the remainder of the membrane. Such skinned microporous membranes are disclosed, for example, in U.S. Pat. No. 3,876,738. Such skinned membranes can be considered to comprise the skin which provides the primary sieving action and a backing layer which is that portion of the membrane which has not formed the skin and has a larger pore size than the pores penetrating the skin. These skinned membranes, however, suffer from at least the same difficulties and problems as the skinless microporous membranes discussed above and can exhibit even higher pressure drops and other poor filtration characteristics. U.S. Pat. No. 4,595,503 attempts to improve the strength and pore size of such skinned microporous membranes by stretching the membranes in at least one direction prior to drying them; however, such stretching can easily result in unacceptably large pores or tears.
In an attempt to avoid that particular problem, U.S. Pat. No. 4,770,777 discloses a somewhat different process of preparing a supported skinned microporous membrane, but the disclosed process does not cure all of the problems attendant skinned microporous membranes. The process involves casting a first membrane layer onto a solid support which is subsequently separated from the first membrane layer, embedding a fabric support into that first membrane layer to form a first membrane layer/fabric support composite, and casting a second membrane layer on top of the first membrane layer/fabric support composite to form a sandwich-like composite. The thus prepared supported skinned microporous membrane, however, suffers from same disadvantages as other skinned microporous membranes with respect to high pressure drop and other poor filtration characteristics.
In particular, significant defects in the form of, for example, macrovoids, cracks, pinholes, and other defects and imperfections that either breach the skin layer or lead to failure upon use, can be present in the membranes. The presence of such defects in the skin can result in a membrane which is rated for the removal of materials of a certain size, but which nevertheless allows for a portion of such material to pass through the membrane upon use. Further, such skinned membranes possess a low level of structural integrity and are easily fouled by debris.
Accordingly, there exists a need for a microporous membrane which is able to provide a high level of structural integrity without a significantly increased pressure drop across the membrane. Moreover, such a membrane should also provide relatively high and uniform titer reduction and, in addition, be substantially defect-free so as to minimize the potential for failure during use.
The present invention provides such a supported microporous membrane which exhibits a high titer reduction, low pressure drop, lack of defects, and good structural integrity. The present invention also provides a method for preparing such a membrane. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.